In various recent read/write apparatuses and communications apparatuses, research and development has been actively conducted on trellis coded modulation (TCM), which is one effective means for reducing the code error rate of digital transmission signals. TCM is a scheme that combines convolutional coding or block coding, both of which take into consideration the characteristics of channels, with decoding such as Viterbi decoding using a trellis diagram based on a state transition diagram of a code. A coding rule is used by a code detector to calculate the likelihood. By increasing the minimum Euclidean distance dmin between trellis code sequences used by the code detector, the error rate of digital transmission signals is reduced.
FIG. 1 is a block diagram of a digital signal processing circuit in a general read/write apparatus.
An encoder 1 encodes rate m:n input data where m represents the data bit length of unencoded data (prior to encoding) and n represents the data bit length of encoded data (subsequent to encoding). A D/A converter 2 converts the input write code from a digital signal into an analog signal, that is, a write rectangular wave. A read/write unit 3 includes a magnetic head, an optical pickup, a control circuit for controlling the driving of the magnetic head and the optical pickup, and the like. The read/write unit 3 writes the write wave input from the D/A converter 2 in a recording medium (not shown).
The read/write unit 3 reads a signal recorded in the recording medium and outputs an analog read wave to an analog equalizer 4. The analog equalizer 4 equalizes the read wave input from the read/write unit 3 to predetermined target equalization characteristics. An A/D converter 5 converts the input analog equalized wave into an equalized signal, that is, a digital read signal. Recently, maximum likelihood detectors are generally used to serve as a code detector 6. The code detector 6 converts the input digital read signal, that is, the equalized signal, into a code. In other words, the code detector 6 detects a code. A decoder 7 decodes the input detected code at a rate of n:m, generates output data, and outputs the output data.
The A/D converter 5 includes a phase locked loop (PLL). As the PLL, a hybrid digital PLL that only performs phase error detection by a digital unit or a full-digital PLL that performs both phase error detection and signal synchronization by a digital unit may be used.
When equalization by the analog equalizer 4 is insufficient, a digital equalizer may be provided between the A/D converter 5 and the code detector 6.
In code detection using TCM, a coding rule used by the encoder 1 is used by the code detector 6.
Recently, TCPR (trellis Coded Partial Response) that combines TCM and partial response equalization has been extensively discussed to serve as signal processing for magnetic read/write apparatuses. TCPR is a technique for increasing the free squared Euclidean distance by an encoding method and consequently improving the S/N (Signal/Noise: signal-to-noise ratio), whereby high-density recording is made possible. Hereinafter the free squared Euclidean distance may also be referred to as the free distance.
The free distance refers to the minimum Euclidean distance between two different paths, both of which originate in a common state and end in a common state on a trellis diagram representing an output sequence of a partial response channel. The trellis diagram is also referred to as a detection trellis. Viterbi detection is performed on the basis of the trellis diagram. The start state and the end state need not be the same.
One known type of trellis code for use in TCPR modulation is an MSN (Matched Spectral Null) code in which a null point of a code spectrum on the frequency axis matches a null point of a signal spectrum that has been partial-response-equalized by limiting a running digital sum (RDS) or an alternating digital sum (ADS) of a code sequence or both RDS and ADS to finite values.
The characteristics of the MSN code and a code detection method therefor are described in detail by, for example, R. Karabed and P. Siegel in “Matched Spectral-Null Codes for Partial-Response Channels,” IEEE Trans. on Info. Theory, vol. 37, No. 3., PP. 818–815, May 1991.
RDS is computed by allocating +1 and −1 to symbols “1” and “0” of a code and computing the sum of symbols from the start point, that is, the initial point, of the code sequence. A code whose digital sum variation (DSV) of RDS is limited to a finite value has a spectral null at a DC component of the power spectrum of the code.
Such a code is known as an MSN code for a dicode channel having 1-D equalization characteristics wherein D is a delay element representing a one-bit delay on the frequency axis. A code whose DSV is limited to a finite value is not limited to the MSN code and is generally referred to as a DC free code. In this case, DSV is two or greater.
For example, the minimum squared Euclidean distance dmin2 of a code detection trellis for the dicode channel is 2. By using a code detection trellis combining the DSV limitation rule and 1-D equalization characteristics, if 2<DVS, equation (1) holds true; if 2=DSV, equation (2) holds true:dmin2=4  (1)dmin2=6  (2)
When duncoded represents the minimum Euclidean distance of unencoded data prior to encoding by the code detector 6, and dcoded represents the minimum Euclidean distance of encoded data subsequent to encoding by the code detector 6, the gain of coding by the code detector 6, that is, the coding gain, is expressed by equation (3):20 log(dcoded/duncoded)dB  (3)
In this case, if 2<DSV, the coding gain is 3 dB; if DSV=2, the coding gain is 6 dB.
Putting the 1-D equalization into practical use without modification is difficult because the 1-D equalization involves very large noise amplification in the high frequency band. In many cases, obtaining a satisfactory signal-to-noise ratio is difficult.
ADS involves, in NRZ modulation, allocation of +1 or 0 to each symbol in a code sequence, multiplication of every other bit by −1, NRZ modulation of the code sequence, and computation of the sum of all symbols from the start point. In NRZI modulation, NRZI modulation is performed on all inverted symbols, and the sum of all symbols from the start point is computed. A code whose A-DSV (Alternating Digital Sum Variation) of ADS is set to a finite range has a null at a Nyquist frequency component of the power spectrum of the code.
Such a code is known as an MSN code on a channel having equalization characteristics of (1+D)x (where x is a natural number). A code whose A-DSV is set to a finite range is not limited to the MSN code and is generally referred to as a Nyquist free code. In this case, A-DSV is two or greater.
(1+D)x equalization is known as being capable of achieving a satisfactory signal-to-noise ratio in read/write apparatuses since noise in the high frequency band of a read signal is suppressed. (1+D)x equalization in a case in which, for example, x=1, that is, 1+D equalization, is generally referred to as PR1 (Partial Response Class-I) equalization. PR1 equalization is put into practical use in, for example, magnetic read/write apparatuses for 3.8-mm streaming tapes and 8-mm streaming tapes.
Such partial response classification was done by Kretzmer. In general, partial response classification that complies with the contents of “Generation of a Technique for Binary Data Communication” by E. R. Kretzmer, IEEE Trans. on com. Tech., pp. 67–68, February 1996 is used.
For example, the minimum squared Euclidean distance dmin2 of a code detection trellis on a PR1 channel is 2. By using a code detection trellis combining A-DSV and PR1 equalization characteristics, if 2<A-DSV, equation (4) holds true; if A-DSV=2, equation (5) holds true:dmin2=4  (4)dmin2=6  (5)
(1+d)x equalization requires a DC component. When this equalization is used in a magnetic read/write apparatus that does not transmit a DC component, generally the DSV of a code must be limited to a finite value in order to prevent degradation of the signal-to-noise ratio of the system. Generally in an optical read/write apparatus that transmits a DC component, the DSV of a code must be similarly limited to a finite value in order to stabilize the PLL and to detect a servo signal.
Codes whose DSV and A-DSV are both limited to finite values have spectral nulls at DC and Nyquist frequency points. Such codes are not limited to MSN codes and are generally referred to as DC- and Nyquist-free codes. In other words, when using MSN codes, magnetic read/write apparatuses employing (1+d)x equalized channels use DC- and Nyquist-free codes.
PR1 MSN codes have already been put into practice by AIT2 (Advanced Intelligent Tape drive II) systems. AIT2 is the standard for 8-mm streaming tape magnetic read/write apparatuses.
DC- and Nyquist-free codes are known as MSN codes for channels with (1−D)(1+D)x equalization characteristics. Since (1−D)(1+D)x equalization suppresses noise in both the low-frequency band and the high-frequency band, including DC, of a read signal, such (1−D)(1+D)x equalization achieves a satisfactory signal-to-noise ratio in read/write apparatuses, and especially in magnetic read/write apparatuses that do not transmit DC components.
In (1−D)(1+D)x equalization, since x=1, that is, (1−D)(1+D), 1−D2 equalization is generally referred to as PR4 (Partial Response Class-IV) equalization. The details of PR4 that has spectral nulls at both DC and Nyquist frequency points are described by Lyle J. Fredrickson in “On the Shannon Capacity of DC- and Nyquist-Free Code”, IEEE Transactions on Information Theory, Vol. 37, No. 3, pp. 918–923, May 1991. PR4 has already been put into practice by hard disk drives, consumer digital VCRs (Video cassette recorders), and the like.
For example, the minimum squared Euclidean distance dmin2 of a code detection trellis on the PR4 channel is 2. Using a code detection trellis combining the DSV and A-DSV limitation rules and PR4 equalization characteristics, dmin2 is 4 or 6, as in the case of the dicode channel.
When both the DSV and A-DSV limitation rules are reflected in the code detection trellis, the trellis detector becomes very complicated. Observation on an every-other-bit basis of a PR4 equalized signal having 1−D2 equalization characteristics gives 1−D equalization characteristics. Generally, a PR4 MSN code is easily constructed by interleaving a dicode channel MSN code on a bit-by-bit (bitwise) basis, assuming that the code is distinctively detected on a bit-by-bit basis.
When the dicode channel MSN code, which is a DC free code, is interleaved on a bit-by-bit basis, a write code is a DC- and Nyquist-free code in which both DSV and A-DSV are limited. In this case, null points of the code spectrum perfectly match null points of the PR4 equalization spectrum. The code is the PR4 MSN code.
In other words, the following method is generally used. When an MSN code is used on the PR4 channel, a dicode channel MSN code in which DSV is limited to a finite value is interleaved on a bit-by-bit basis, and a resulting code is recorded on a recording medium. When reading the code, the equalized signal is de-interlieaved on a bit-by-bit basis to detect the code.
As described above, although MSN codes obtain high coding gain, PR4 codes are difficult to put into practice in actual apparatuses.
One cause of this problem is that a known method employs a low coding rate of 4/5 and that the signal-to-noise ratio is greatly degraded in the case of a high linear recording density. Such a known method is thus inapplicable in practice. Recently, however, as disclosed in Japanese Unexamined Patent Application Publication No. 11-186917 submitted by the inventor of the present invention and in “High-Rate Matched Spectral Null code” by M. Noda, IEEE Trans. on Magn., vol. 34, No. 4, pp. 1946–1948, July 1998, this weakness is improved by technology concerning MSN codes having a high coding rate of 8/9.
Another cause of the problem of the PR4 MSN code being difficult to put into practice is that the maximum magnetization reversal interval Tmax is increased when the code is recorded. In the case of NRZI recording, the maximum magnetization reversal interval Tmax when the code is recorded is the maximum length of a continuous sequence of data 0 (the maximum run-length)+1. In the case of the PR4 MSN code, the maximum magnetization reversal interval Tmax is increased since two independent dicode channel MSN codes are interleaved on a bit-by-bit basis. In other words, the maximum magnetization reversal interval Tmax of an interleaved code (subsequent to interleaving) is twice as high as the maximum magnetization reversal interval Tmax of a non-interleaved code (prior to interleaving).
In the case of NRZI recording, the maximum magnetization reversal interval Tmax is the sum of the maximum length of a continuous sequence of data 0 (the maximum run-length) and 1. In the case of NRZ recording, the maximum magnetization reversal interval Tmax is equivalent to the maximum run-length.
For example, in magnetic reading apparatuses, a reduction in Tmax of a code has a great favorable influence on the reduction in overwrite noise of a read signal and on PLL stabilization. Specifically, when Tmax of a code is high, information for achieving PLL synchronization is reduced, which may cause a failure.
In the case of a high Tmax, azimuth recording generates a high cross-talk from neighboring tracks, which in turn degrades the quality of read data. As discussed above, a reduction in Tmax of a code is widely known to help improve the performance of read/write apparatuses.
In the read/write apparatuses, especially a helical-scan tape system using a rotary drum, a high-pass filter is formed by a winding inductance of a rotary transformer and a winding DC resistance of a write head. The low frequency components of a recording current waveform are cut off, and it is difficult to write a recording waveform signal with a high Tmax on a magnetic tape.
In general read/write apparatuses, Tmax of a code to be used is generally designed such that approximately Tmax≦9T where T denotes the clock time interval. The above-described rate 8/9 dicode channel MSN code has a Tmax of 7T. In contrast, when this code is interleaved on a bit-by-bit basis, Tmax of the resultant code is twice as high, that is, 14T. In other words, since the two independent dicode channel MSN codes are interleaved on a bit-by-bit basis, Tmax becomes very high, which is inapplicable in practice.